Amoebae are single-celled organisms found in water and animals. They are protists with a flexible cell membrane, containing cytoplasm, a nucleus, contractile and food vacuoles. They use pseudopods for mobility and feeding, and reproduce asexually. Some species cause disease in humans, such as amoebic dysentery and primary amoebic meningoencephalitis. Amoebae are fascinating to observe under a microscope and can be found in freshwater and mud.
An amoeba is a type of single-celled organism usually found in water around decaying vegetation, moist soil, and in animals such as humans. It is relatively advanced and can extend and retract blob-like arms called pseudopods, which it can use to grab food particles and propel itself through microscopic terrain. The name “amoeba” can mean a particular genus of microorganism or any member of a large family of similar life forms. While most types are harmless, some can cause serious illness in humans.
Protists
These organisms are types of protists, which are a group of simple life forms that are neither plants nor animals. Most are microscopic, but some species are large enough to be visible to the naked eye. The amoeba proteus is the best known species and measures about 0.028 inches (0.7 mm), which is barely visible. Some species can reach 0.1 inch (about 3 millimeters) or more in diameter. German naturalist August von Rosenhof is thought to have been the first to observe these microorganisms in 1757, using an antique microscope.
Structure
It is easy to study amoebae because they are relatively large and nearly transparent. A basic light microscope can then reveal their internal structure and organs. The mobility and complexity of the organism make it much more interesting to study than the more simplistic protists. Scientists often use the amoeba for testing and observation, calling it a “model organism.”
An amoeba consists of a thin, flexible cell membrane containing a fluid called cytoplasm and various organs. The nucleus contains the genetic material. The contractile vacuole is used to regulate the amount of water in the cell and this important organ can absorb water from the cytoplasm and expel it from the cell through the membrane. The food vacuoles are used to store and digest the food that the amoeba has absorbed.
Habits and life cycle
Many species of amoebae are found in freshwater, usually in mud at the bottom of pools or among decaying vegetation. Others live in the human digestive system. These include those that can cause disease, but most simply live on food in the gut without causing any problems. Some types live on the skin, feeding on dead skin cells.
Amoebae use their pseudopods for mobility and for obtaining food. They can use these extensions to engulf small particles or smaller organisms and incorporate them into the cell, where they are digested. Most species feed on tiny bits of organic matter and other microorganisms, such as bacteria. Some are parasites that devour the cells of the host animal.
Depending on the salt content of the water the organism is in, it will shrink or swell in an attempt to equalize its density with that of its surroundings. If one is placed in water that contains no salt at all, it can swell so much that it bursts. When an amoeba encounters harsh conditions, such as drought or cold temperatures, it can shrink and surround itself in a hard, protective coating. This is known as a cyst. It can exist for long periods in this state until conditions improve, but it may not survive indefinitely.
Amoebas reproduce asexually by dividing into two “daughter” cells. The nucleus divides first, forming two genetically identical nuclei, a complex process known as mitosis. The body then divides into two separate cells, each containing a nucleus, a process called cytokinesis.
Amoebas and disease
Some amoebas can cause disease in humans, the best known being amoebic dysentery, or amoebiasis, caused by the organism Entamoeba histolytica. It is transmitted through water contaminated by sewage. Humans can be exposed to it by eating food that has come in contact with contaminated water and has not been properly washed. The organism attacks the intestines, causing pain and diarrhea. The disease is very unpleasant and can be serious, but it is treatable.
Primary amoebic meningoencephalitis (PAM) is an extremely serious but fortunately very rare brain disease caused by an organism called Naegleria fowleri, sometimes called the “brain-eating” amoeba. The infection is almost always fatal. The organism lives in lakes and warm pools in many parts of the world where water temperatures are high enough, including the southern states of the United States and Australia.
People who swim or play in water containing the organism are at risk of becoming infected. It enters the body through the nostrils, migrating to the brain, where it consumes cells, causing brain tissue damage and inflammation. The best way to prevent the disease is to avoid likely habitats such as quiet, warm pools or to wear nose plugs if swimming in such areas.
Observing amoebas
For anyone with a microscope, these microorganisms make a fascinating subject to observe. However, they are sometimes hard to find. One way to obtain live specimens is to use a jar to skim the top layer of mud off the bottom of a pond. Another method is to take a sample of the pond water, including some plants, and place a coverslip on the surface so it floats. After about a day, the bacteria will attach themselves to the undersurface, attracting small predators, including amoebae.
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